Problem: On the first 3 exams of his math class, Omar got an average score of 87. What does he need on the next exam to have an overall average of 89?
Explanation: Let his score on the next exam be $x$ The sum of all of his scores is then $3 \cdot 87 + x$ The same sum must also be equal to $4 \cdot 89$ Solve: $x = 4 \cdot 89 - 3 \cdot 87 = 95$.